Polylactic acid resins that are biodegradable polyester resins are expected to be used as molding materials in various fields, for example, packaging materials such as containers and films, fibrous materials such as clothing, floor mats, and automobile interior materials, housings and parts of electrical and electronic products, and the like.
In order to improve the molding processability and the heat resistance of a polylactic acid resin, attempts have been made to enhance the crystallization rate and the degree of crystallinity of the resin and, as a method for achieving it, addition of a nucleating agent has been developed. The nucleating agent serves as a primary crystal nucleus of a crystalline polymer to facilitate crystal growth, to reduce the crystal size, and to increase the crystallization rate. As nucleating agents for polylactic acid resins, an inorganic particle that includes talc or boron nitride of not larger than a certain particle diameter (Patent Document 1), an amide compound of a certain formula (Patent Document 2), a sorbitol derivative of a certain formula (Patent Document 3), a phosphoric acid ester metal salt of a certain formula (Patent Document 4), phosphonic acid metal salts (Patent Document 5 and Patent Document 6), and the like have been disclosed.
Phosphonic acid metal salts are considered to have excellent performance among these nucleating agents, and are usually produced by causing a reaction of a phosphonic acid-based compound with a metal ion source, for example, a metal hydroxide, a metal oxide, a metal nitrate, or a metal acetate, in water or an organic solvent.
For enhancing the crystallization rate and the degree of crystallinity of the polylactic acid resin, for example, the size of the nucleating agent is reduced. Generally, the smaller the size of the nucleating agent is, the greater the number of particles and the surface area per mass are. In addition, the finer the nucleating agent is, the smaller the size of a polylactic acid resin crystal is. When the particle size of the nucleating agent is small, the transparency of a resin product is enhanced, namely the performance of the nucleating agent is enhanced.
For example, regarding the phosphonic acid metal salt described in Patent Document 5, it is described that pulverization treatment or the like is carried out where appropriate so as to achieve an average particle diameter of not larger than 10 μm. It is also described in Example of the same document that a phosphonic acid metal salt with an average particle diameter of 1.1 μm at the smallest is produced.